1. Field of the Invention
The disclosed subject matter relates to a flue gas processing system employing a catalyst to remove or reduce the amount of nitrogen oxides (NOx) released from the flue gas processing system. More particularly, the disclosed subject matter relates to a method of protecting the catalyst from fouling.
2. Description of Related Art
Burning of carbonaceous fuels results in generation of many byproducts, including, but not limited to carbon monoxide (CO), hydrocarbons, soot, nitrogen oxides (NOx), sulfur oxides (SOx) and the like. In the United States, release of such byproducts into the environment is tightly regulated by various federal and state laws and regulations. Accordingly, technology that reduces or eliminates the emission of CO, hydrocarbons, soot, NOx, SOx and the like, have been developed and introduced to process the exhaust gases (referred to as “flue gas”) containing these byproducts.
Flue gas treatment techniques that reduce or eliminate NOx emissions typically employ various chemical or catalytic methods. Methods include non-selective catalytic reduction (NSCR), selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) (hereinafter collectively referred to as “NOx reducing catalysts”). Alternatively, NO may be oxidized to NO2 for removal by wet scrubbers.
The NSCR method typically uses unburned hydrocarbons and CO to reduce NOx emissions in the absence of O2. Chemical reactions on a solid catalyst surface of SCR systems convert NOx to N2. Commercial SCR systems typically use ammonia (NH3) as the reductant. SCR technology generally involves injecting ammonia into the flue gas and passing it over a catalyst where the ammonia reacts with NOx to produce molecular nitrogen and water vapor.
NOx reducing catalysts are often completely or partially deactivated when exposed to flue gas (oftentimes referred to as “fouling”). Partial or complete deactivation of the catalysts occur when calcium deposits, commonly calcium oxide, become sulfated and form calcium sulfate, thereby plugging the pores of the catalyst and interfering with the reduction of NOx. Accordingly, only certain fuels and boilers or furnaces are used in conjunction with a NOx reducing catalyst.
Partial or complete deactivation of NOx reducing catalysts may result in increased NOx emissions, partial or complete plant shut down, or replacement of the catalyst. Such drawbacks increase interruptions to plant productivity which may lead to a decrease in the efficiency of the plant as well as an increase in costs of running the plant.
Placement of a particulate removal device prior to the NOx reducing catalyst may slow down the deactivation of the NOx reducing catalyst. However, particulate removal devices increase construction and operating expenses of the flue gas processing system. Moreover, retrofitting a particulate removal device and/or a NOx reducing catalyst in a system to decrease NOx emissions is a costly endeavor due to system and operation re-design.